1. Field of the Invention
The present invention relates generally to circuits for effecting wavelength stabilization in optical communication systems that include lasers for producing optical signals carrying data and noise. More specifically, the invention relates to circuits and methods for increasing the signal-to-noise ratio in optical communication systems to stabilize laser wavelengths in the system.
2. Description of the Related Art
Lasers are commonly used in optical communication systems for multiplexing data channels. In such systems, multiple lasers produce signals having different but very close frequencies so that multiple channels of data can be transmitted by the system. The achievement of wavelength or frequency stabilization of the lasers has been an inherent problem in such systems, and has taken on greater importance for both wavelength division multiplexed (WDM) and dense wavelength division multiplexed (DWDM) optical communication systems as the need to increase the number of channels has proliferated. DWDM systems in particular experience stabilization problems since the spacing of channels is reduced to 100 GHz or less to achieve greater system capacity. Moreover, it is necessary to realize high signal-to-noise ratios in order to effectively achieve frequency stabilization in optical communication systems.
In a stabilized optical communication system, wavelength or frequency drift can nevertheless be unintentionally and undesirably introduced by the aging and/or temperature dependence of the laser itself, and/or by the stabilization electronics. The wavelength of a laser in such an optical communication system is typically controlled by temperature. Thus a thermoelectric cooler (TEC) is placed in thermal contact with the laser to control the laser""s temperature via a feedback current. The temperature of the TEC is varied based on the level of the feedback current to thereby change the temperature of the laser. In theory, this feedback loop stabilizes the laser""s wavelength. Still, drift is rampant in such systems and causes system performance to degrade. One proposed solution is to split the laser light into two paths, at least one of which passes through an optical filter; the second path defines a power reference that is used for optical power normalization. This approach is taught and disclosed in commonly owned copending U.S. patent application Ser. No. 09/265,291, filed Mar. 9, 1999, the teachings of which are incorporated herein by reference. After normalization, a signal is obtained that is essentially a function of only wavelength and is therefore used to stabilize the laser. Another approach is to add a modulation circuit in each of the paths after the control signal is detected to produce a modulated control signal for use in stabilization. This approach is taught and disclosed in commonly owned copending U.S. patent application Ser. No. 09/140,050, filed Aug. 26, 1998, the teachings of which are incorporated herein by reference.
While the two path approach lends itself well to optical systems, because the overall aging factors of all of the components, especially the electronics, in the system are unknown it is anticipated that it may be difficult to obtain correct optical power normalization. This deficiency requires the additional introduction of autozeroing and other reduction techniques to the system, which are both time consuming and costly. A solution of this nature is taught and disclosed in commonly owned U.S. patent application Ser. No. 09/265,338, filed Mar. 9, 1999 the teachings of which are incorporated herein by reference. Additionally, the two path approach requires two stable paths for the channels or that a stable multiplexer be provided to a microcontroller which implements the feedback path to control the laser. Furthermore, adding modulators in each of the paths may be uneconomical and inefficient since it simply adds additional components that themselves contribute to the overall aging problems associated with the circuit in the first instance.
Moreover, it is particularly important in DWDM systems that, after detection of the optical signals and amplification, a high signal-to-noise ratio be achieved in order to ensure that adequate wavelength stabilization will be attained. In the past, a high signal-to-noise ratio has been obtained by reducing the input capacitance which appears at the front end of an amplifier that amplifies the electrical signal output from a detector which converts the optical signal to a usable analog electrical signal containing representations of the data and noise components found in the system. However, this approach has typically been applied in wide bandwidth applications where the capacitance term can dominate the noise contained on the communication signals. For wavelength stabilization, a low bandwidth portion of the spectrum may be used. The inventors herein have recognized that contrary to the usual wide bandwidth case, decreasing input capacitance will actually contribute to the noise current and create deleterious effects on the system when wavelength stabilization is desired, since less filtering is achieved.
Accordingly, there is a long-felt, but unresolved, need in the art for systems for stabilizing the wavelength of laser controlled elements of optical communication systems. The systems should be cost efficient and easily implemented in existing systems. It would be desirable if such systems did not introduce high-cost or additional hardware elements to the circuits that implement the systems in order to save costs and to preserve the precious physical space of already densely packed optical communication systems. Such needs have not heretofore been met in the optical communications art.
The aforementioned problems are solved, and long felt needs met, by circuits and methods of the present invention for stabilizing a wavelength of an optical signal carrying noise and data components in an optical communication system. The inventive circuits achieve wavelength stabilization by increasing the signal-to-noise ratio of a control signal obtained by conversion from the optical signal, wherein the electrical signal includes electrical representations of the noise and wavelength of the optical signal. The circuits include a receiver for receiving the electrical signal and amplifying the electrical signal so that the amplified electrical signal can be used in a feedback loop to create a wavelength-stabilized electrical control signal that controls the output wavelength of the laser. A feedback resistor is connected to an output node of the receiver and controls a gain of the receiver. An impedance element is coupled to an input node of the receiver and modifies the characteristic impedance of the circuit, thereby filtering noise from the electrical signal and increasing the signal-to-noise ratio at the input node of the receiver so that the receiver outputs the amplified electrical signal which is used to create the electrical control signal for wavelength stabilization.
In a preferred embodiment, the noise signal is a current whose magnitude is dependent on input capacitance at wide bandwidth. But at low bandwidths, decreasing the bandwidth will increase the signal-to-noise ratio by reducing the noise current. In a further preferred aspect of the invention, the impedance element used to reduce the noise bandwidth is a capacitor.
The circuits of the present invention ensure that high signal-to-noise ratios are achieved so that wavelengths in optical communication systems in which the circuits are employed can be easily and efficiently stabilized. The inventors of the circuits and methods disclosed and claimed herein have recognized that, in contrast with high bandwidth applications, the total noise current in low bandwidth systems can be reduced by the addition of capacitance which reduces the bandwidth. This increases the signal-to-noise ratio of the system, thereby effectively achieving improved wavelength stabilization of the laser. Such results have not heretofore been achieved in the art.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.